Lazyweb: citation for "stored globals have stable addresses"?

[jrose (Jordan Rose)]

In writing my blog series (Blog series: The Swift Runtime), I want to cite when I'm relying on a property that I and others have talked about before: the inout-to-pointer conversion for a stored global will always use the actual address of the global instead of making a local copy. This guarantee is only provided for globals and static stored properties right now, and even then only in modules compiled without library evolution support (plus non-const globals imported from C, of course).

I know we want to do better here, with @lorentey and others having mentioned a hypothetical "addressable" attribute that could be applied to class instance properties (and elsewhere?), and the previous thread "Passing an extern const C struct by pointer possible?". However, right now I'm just looking for a citation (Apple or swift.org docs, WWDC presentation, compiler docs) of the guarantee I stated above.

Here's the earliest forum post I can find (from me, actually):

KVO Context Param

Hi, Tyler. Swift doesn't guarantee stable addresses for anything but top-level and static variables, so you'd have to declare a 'var' to make a valid KVO context anyway. It's not a great situation, but it's unlikely to change any time soon. Jordan

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On Oct 6, 2016, at 14:07, Tyler Stromberg via swift-dev <swift-dev@swift.org> wrote: Hey everyone, One of the things that came up in a code review today was the type of KVO's context param. If you look at the function signature it gets…

But that's not really a good citation in the same way as "Guarantee (in-memory) tuple layout…or don't" doesn't have a good citation. Anyone know of something better?

(see also The state of Swift documentation :-( )

[owenv (Owen Voorhees)]

It's not spelled out as clearly as it could be, but https://github.com/apple/swift/blob/master/userdocs/diagnostics/temporary-pointers.md touches on this:

Not all inout-to-pointer conversions result in a temporary pointer. Passing global variables and static properties inout can produce non-ephemeral pointers, as long as they are stored and have no observers. Additionally, if they are of a tuple or struct type, their stored members without observers may also be passed inout as non-ephemeral pointers.

[Joe_Groff]

Note that even then, only the physical value of the pointer is guaranteed to be the same. You can't load or store from a pointer to a global outside of the with* block or pointer-to-inout call that created the pointer.